Adenylate cyclase lead inhibition

Many of the adverse effects of lithium can be ascribed to the action of lithium on adenylate cyclase, the key enz)nne that links many hormones and neurotransmitters with their intracellular actions. Thus antidiuretic hormone and thyroid-stimulating-hormone-sensitive adenylate cyclases are inhibited by therapeutic concentrations of the drug, which frequently leads to enhanced diuresis, h)rpoth)n oidism and even goitre. Aldosterone synthesis is increased following chronic lithium treatment and is probably a secondary consequence of the enhanced diuresis caused by the inhibition of antidiuretic-hormone-sensitive adenylate cyclase in the kidney. There is also evidence that chronic lithium treatment causes an increase in serum parathyroid hormone levels and, with this, a rise in calcium and magnesium concentrations. A decrease in plasma phosphate and in bone mineralization can also be attributed to the effects of the drug on parathyroid activity. Whether these changes are of any clinical consequence is unclear. 

In the case of the P2-catecholamine receptor (illustrated here), the a-subunit of the Gs protein, by binding to adenylate cyclase, leads to the synthesis of the second messenger cAMP. cAMP activates protein kinase A, which in turn activates or inhibits other proteins (2 see p.l20). 

Nathanson and Bloom (1975) showed that brain adenylate cyclase is inhibited in vitro by lead at very low concentrations. An IC50 value was obtained of 3/tmol/l (equivalent to 62 g/100ml or 0.62 ppm). Inhibition might take place in the brain of normal rats, since Sauerhoff and Michaelson (1973) have shown brain lead concentrations of 0.1 ppm. These authors also reported that lead-exposed rats have brain leads of 0.6-1.2 ppm which caused inhibition of adenylate cyclase of above 50% in the parallel in vitro system. 

The OP group of receptois share common effector mechanisms. All receptois couple via pertussis toxin-sensitive Go and Gi proteins leading to (i) inhibition of adenylate cyclase (ii) reduction of Ca2+ currents via diverse Ca2+ channels (hi) activation of inward rectifying K+ channels. In addition, the majority of these receptors cause the activation of phospholipase A2 (PLA2), phospholipase C 3 (PLC 3), phospholipase D2 and of MAP (mitogen-activated protein) kinase (Table 3). 

ADP ribosylation results in inhibition of GTPase activity and hence maintains the a-subunit in the active form. The constant activity of the G-protein results in an increase in adenyl cyclase activity and therefore a chronic increase in the cychc AMP level. This stimulates an ion channel in the enterocyte which results in a loss of Na ions and hence water from the cells into the intestine. This leads to diarrhoea and a massive loss of fluid from the body which can be sufficiently severe to result in death. Since 2000 there have been epidemics in South America and parts of central Africa. Infection is usually caused by drinking water contaminated with faecal matter. Treatment consists of hydration with rehydration fluids (Chapter 5). 

The resulting failure ofG, to inhibit adenylate cyclase increases cyclic AMP in airway cells and leads to fluid imbalance and the severe, life-threatening congestion of whooping cough. 

Similar to a P-adrenoceptor stimulation intracellular cAMP can be increased by inhibition of phosphodiesterase. Thus, in turtle retina cells, cAMP leads to uncoupling and this can be mimicked by stimulation of adenylate cyclase with forskolin and concomitant inhibition of phosphodiesterase by IBMX [Piccolino et al., 1984]. In cardiac cells inhibition of phosphodiesterase has been investigated using methylxanthine derivates [De Mello, 1989], resulting in an enhancement of intercellular coupling. 

Activation of Gq and Gs leads primarily to facilitation of transmitter release, along a slower and more prolonged time-scale than G0-mediated inhibition. This results predominantly from downstream products of Gq and Gs enzyme activation - diacylglycerol (DAG) from Gq-activated phospholipase C (PLC) and cyclicAMP (cAMP) from Gs-activated adenylate cyclase (AC). 

Fig. 2 Mechanisms involved in presynaptic facilitation through A2 adenosine receptors. A2A and A2B adenosine receptors (A2aAR, A2B AR), by coupling to Gs, activate adenylate cyclase and protein kinase A (PKA). This may (1) influence SNARE proteins or (2) enhance calcium currents through P-type voltage-sensitive calcium channels (P-VSCC). A2aAR may also couple to Gq, leading to activation of a protein kinase C (PKC) pathway. This may (3) enhance calcium currents through N-VSCC, (4) influence SNARE proteins, (5) promote the PKA pathways or (6) remove an ongoing Gj/0 mediated inhibition of release.

As Illustrated in Fig. 7, 3 yM CRF and 1 yM (-)Isoproterenol cause a 190 and 110% stimulation of adenylate cyclase activity In rat pars intermedia particulate fraction, respectively. An additive effect Is observed when both stimulatory agents are present. Dopamine (30 yM), on the other hand, has no significant effect alone. However, In the presence of GXP, the catecholamine causes a 40 to 60% Inhibition of adenylate cyclase activity stimulated by CRF, ISO or CRF + ISO. It can also be seen that while 0.3 mM GXP alone causes a 100% increase In basal adenylate cyclase activity, it leads to a marked potentiation of the effect of ISO and CRF on [ 2P] cyclic AMP accumulation. It should be noticed that In the absence of the guanyl nucleotide, dopamine has no Inhibitory effect on adenylate cyclase activity In any of the groups studied. 

Sofar (presynaptic) dopamine receptors mediating inhibition of dopamine turnover and/or release seem to display features resembling those of the D-2 receptor. However inconsistencies reported could finally lead to a subdivision of D-2 receptors in the near future or it will appear that some in vitro experimental conditions have been too extreme, which by itself would have induced changes in the pharmacological characteristic of the receptor. One obvious question emerging from this review is whether all D-2 dopamine receptors in the neostriatum are linked to an adenylate cyclase. With the methodology presently available it will be hopefully only a matter of time to answer this question. 

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